In recent years, various optical devices such as an optical switch, an optical multiplexer/demultiplexer and an optical modulator have been experimentally produced using an optical waveguide with a core of a semiconductor such as Si and Ge and a compound semiconductor such as AlGaAs and InGaAsP. An optical waveguide device using Si for a core can be significantly downsized compared with a conventional optical waveguide device using silica for a core, and furthermore, the characteristics thereof can be electrically controlled in an active manner by injecting a current and applying a voltage to the core itself because the core is formed of a semiconductor. On the other hand, in general, a conventional optical waveguide using silica for a core is thermally controlled. This is because silica is an insulator, so that a current cannot be caused to flow therein. As a method of controlling the characteristics of an optical waveguide such as the propagation loss of light wave, the propagation constant of light wave, group velocity and dispersion, there have been known methods of using any of a thermo-optic (T-O) effect, an acousto-optic (A-O) effect, a magneto-optic (M-O) effect, an electro-optic (E-O) effect (also referred to as “Pockels effect”) and a carrier plasma effect. Among others, the method of using an electro-optic (E-O) effect by which a voltage is applied to vary a refractive index and the method of controlling a refractive index using a carrier plasma effect produced by injecting a current are used for controlling the characteristics at a high speed because the methods can provide quick response in 1 nsec or less.
Incidentally, as an optical waveguide using Si for a core, there exist a Si rib waveguide and an Si wire waveguide. The Si rib waveguide provides an oxide film and a control electrode thereon to form an MOS structure, thereby applying voltage to realize the control of a refractive index of the waveguide. Recently, an Si high-speed optical modulator using the MOS structure including the Si rib waveguide has been realized. However, the Si rib waveguide essentially loosely confines light into a core, so that the Si rib waveguide does not adapt to a sharp curvature with a radius of curvature of several μm. For this reason, the waveguide in the Si rib waveguide needs to be curved gently, which inevitably increases the size of devices such as an optical switch and an optical modulator using the waveguide. On the other hand, the Si wire waveguide strongly confines light into a core and can adapt to a sharp curvature with a radius of curvature of several μm, so that the Si wire waveguide draws attention as a technique by which an optical device can be downsized.
However, the cross section of a core of the Si wire optical waveguide is in the order of sub-micron square and the periphery of the core is normally covered with an insulator such as silica and air, so that it is difficult to uniformly and effectively inject a current into and apply a voltage to the core.
Patent document 1 (Japanese Patent Laid-Open No. 2004-170836) describes a variable optical attenuator in which any optional light attenuation can be provided by an electrical control using a waveguide having a Si wire as a core which is covered with a clad, such as of an insulator. An upper clad layer, which is made of silicon (for example, polysilicon) to which oxygen or nitrogen is added, is formed to cover the upper portion and both sides of the Si core and forms a waveguide along with the core. The waveguide includes a p-type carrier supply section in which a p-type impurity is introduced into a part of the side of the upper clad and an n-type carrier supply section in which an n-type impurity is introduced into the opposing side thereof. Since oxygen or nitrogen is added to the regions where the p-type and n-type carrier supply sections are formed, the periphery of the core is rendered low in refractive index, thereby light is confined in the core.
On the other hand, the following has been known. In a photonic crystal with a periodic distribution of refractive index in the order of the wavelength of light, there exists a so-called photonic band gap in which the presence of light in a wavelength region corresponding to the period is prohibited, and the introduction of an artificial defect disturbing a periodic structure into the crystal enables light to exist in the photonic band gap and various lights to be controlled.
An optical switch with such a photonic crystal structure is described in Patent Document 2 (Japanese Patent Laid-Open No. 2002-303836). In the description in paragraph numbers 0054 to 0060 and FIG. 12 of Patent Document 2, there disclose: a triangular lattice photonic crystal structure and a line-defect waveguide are formed in a non-doped Si layer of an SOI wafer; impurities are injected into both sides of the line-defect waveguide to form electrodes; the photonic band gap structure of the photonic crystal in the part of the line-defect waveguide through which light propagates is varied by which a current can be injected or a reverse bias can be applied; and the waveguide mode of the line defect waveguide existing in the photonic band gap becomes a cut-off state (or, in a state in which light cannot propagate), which disables light from propagating through the line defect waveguide and functions as an optical switch.    Patent document 1: Japanese Patent Laid-Open No. 2004-170836    Patent Document 2: Japanese Patent Laid-Open No. 2002-303836 (paragraph numbers 0054 to 0060 and FIG. 12)